The invention relates generally to insulating systems for electrical machines and machine windings, and more specifically to an insulation system having non-linear dielectric properties.
Electrical machines and devices such as generators, motors, actuators, transformers, etc. are constantly subjected to various electrical, mechanical, thermal, and environmental stresses. Such stresses tend to degrade them, consequently reducing their lives. In an example, a static magnetic field is retained after power is disconnected in a steel core in transformers due to magnetic remanence. When power is further reapplied, residual field causes a high inrush current until effect of the magnetic remanence is reduced, usually after a few cycles of applied alternating current. Overcurrent protection devices such as fuses in transformers connected to long overhead power transmission lines are unable to protect the transformers from induced currents due to geomagnetic disturbances during solar storms that may cause saturation of the steel core, and false operation of transformer protection devices. It has been commonly observed that deterioration of insulation in the foregoing devices is a dominant factor in their failures.
Insulation systems for electrical machines such as generators, motors and transformers have been under constant development to improve performance of the machines. Materials generally used in electrical insulation include polyimide film, epoxy-glass fiber composite and mica tape. Insulating materials generally need to have the mechanical and physical properties that can withstand various electrical rigors of the electrical machines such as lightning and switching surges. In addition, some of the desirable properties of an insulation system include withstanding extreme operating temperature variations, and a long design life.
The aforementioned insulating materials have an essentially constant dielectric constant, which protects them from electrical conduction based on their respective composite breakdown strengths. However, certain factors such as operating temperatures, environment, voltage stresses, thermal cycling and voltage surges from lightning and switching deteriorate the insulating materials over a long period of time thus reducing their useful or operational life.
Therefore, it would be desirable to provide an insulation system that would address the aforementioned problems and meet the current demands of industry applications.